1. Field of the Invention
The present invention relates to a motor rotor in which a rotating balance can be corrected with a good operability, and a method for correcting the rotating balance, and more particularly to a motor rotor which is preferably mounted to a motor-driven supercharger, and a method for correcting a rotating balance thereof.
2. Description of the Related Art
In order to improve an internal combustion engine, there has been widely used a supercharger (called also as “turbocharger”) driven by an exhaust gas of an internal combustion engine so as to compress an intake air and supercharge. Further, there is employed a supercharger in which an acceleration response or the like is improved by embedding an electric motor coaxially with a rotating shaft of the supercharger and accelerating and assisting a rotational drive of a compressor. The supercharger having a motor-driven assist function by the electric motor is called as a motor-driven supercharger.
A description will be given briefly of a structure of this kind of motor-driven supercharger. A supercharger rotor structured such that a turbine impeller and a compressor impeller are coupled to both ends of a rotating shaft is rotatably supported within a housing. The housing has an electric motor built-in. A rotor (a motor rotor) of the electric motor is fixed coaxially with the rotating shaft, and a stator (a motor stator) of the electric motor is arranged around the rotor in an inner portion of the housing. If an exhaust gas from the internal combustion engine is supplied to a turbine impeller, the turbine impeller is rotationally driven, and a compressor impeller coupled to the turbine impeller is rotationally driven, thereby compressing an intake air so as to supply to the internal combustion engine. Further, at this time, a rotational drive of the compressor impeller is assisted by the electric motor.
The motor rotor of the motor-driven supercharger mentioned above is disclosed in the following patent document 1. FIG. 1 is a cross sectional view showing a conventional motor rotor disclosed in patent document 1. The motor rotor is constituted by an inner sleeve 51 inserted and attached to a turbine shaft 50 of a supercharger, a permanent magnet 52 surrounding the inner sleeve 51 around an axis, and a hollow cylindrical outer sleeve 53 surrounding the permanent magnet 52 around the axis. The outer sleeve 53 is shrink fitted in such a manner as to be capable of sufficiently holding the permanent magnet 52 even under a condition that a great centrifugal force is applied at a maximum rotating speed of the rotor.
In a manufacturing step of the motor-driven supercharger, a rotating balance is corrected by executing a rotating balance test after assembling the motor rotor. In the case of the conventional motor rotor shown in FIG. 1, the balance correction is executed by pruning away a part (a portion shown by reference symbol A in the drawing) of an end surface of the permanent magnet 52. However, if the permanent magnet 52 is pruned away, a magnetic force of the permanent magnet 52 is changed. Since a rotating balance correcting amount has an individual difference, a magnetic force of the permanent magnet 52 is dispersed per products. Further, since a crack is generated by pruning away the permanent magnet 52, or a stress generated at a time when a force is applied to the permanent magnetic 52 becomes non-uniform in comparison with the case that the permanent magnet 52 is not pruned away, a strength is lowered.
In order to cope with the problem mentioned above, the conventional additional motor rotor as shown in FIG. 2 has been proposed. The motor rotor is constituted by an inner sleeve 51 inserted and attached to the turbine shaft 50, the permanent magnet 52 surrounding the inner sleeve 51 around an axis, a pair of end rings 54 and 54 pinching the permanent magnet 52 from both sides in an axial direction, and a hollow cylindrical outer sleeve 53 surrounding the permanent magnet 52 and a pair of end rings 54 and 54 around the axis. The outer sleeve 53 is shrink fitted to the permanent magnet 52 and the end rings 54 and 54.
As shown in FIG. 2, the end ring 54 and the outer sleeve 53 are flush in end surfaces. This is because no compression load with respect to the end ring 54 is applied to a protruding portion of the outer sleeve 53 from the end ring 54 and the protruding portion is not necessary functionally.
In the motor rotor having the structure mentioned above, the rotating balance correction is executed by pruning away a part (a portion shown by reference symbol B in the drawing) of the end ring 54. In this case, since it is not necessary to prune away the permanent magnet 52, there is not generated a problem that a magnetic force change or a strength reduction is generated.    Patent Document 1: The U.S. Pat. No. 6,085,527 (FIG. 5)
However, for example, in the motor-driven supercharger for a vehicle, since the motor rotor itself is a comparatively compact part, and the inner sleeve 51 protrudes in an axial direction from the end ring 54 as shown in FIG. 2, the inner sleeve 51 forms an obstacle and it is hard to prune away the end ring 54. In other words, an operability of the rotating balance correction is deteriorated.
As opposed to this problem, there is considered a method of pruning away the end portion in the axial direction of the outer sleeve 53, however, the end portion in the axial direction corresponds to a portion applying the compression load to the end ring 54, so that if this portion is pruned away, a fastening force with respect to the end ring 54 is reduced. Further, if this portion is pruned away too much, there is a possibility that a problem such as the end ring 54 slips (idle runs) or the like is generated. Accordingly, in the structure in FIG. 2, the method of pruning away the end portion in the axial direction of the outer sleeve 53 is not expedient.